Do I collect sales tax on gift certificate purchases?

No.  Sales tax is only applied at the time of sale when the card is used.

Gift cards and certificates are a form of stored value card where sales are deducted from them.  They are a cash equivalent so for the business that issued them, they are considered the same as money.  If the client paid tax on the gift card, and then paid tax on the sale of the service they would be double charged.